There are various types of wells, such as land based wells and offshore wells, for producing oil, gas, water, or hydrates. Offshore wells may also be shallow or deep water wells. A shallow offshore well is typically drilled from a platform that is in water up to 3,000 feet in depth. A deep water well is drilled from a floating platform or vessel with a riser extending from the sea floor to the platform or submersible rig. Any water deeper than 5,000 feet requires a drilling vessel, typically a drill ship.
Well construction, such as the drilling of an oil or gas well, includes a wellbore or borehole being drilled through a series of formations. Each formation through which the well passes must be sealed to avoid an undesirable passage of formation fluids, gases, or materials out of the formation and into the borehole or from the borehole into the formation. It is also commonly desired to isolate both producing and non-producing formations from each other to avoid contaminating one formation with the fluids from another formation.
To seal or isolate the formations, conventional well architecture includes casing the borehole. The formations may also be cased for borehole stability due to the geo-mechanics of the formation such as compaction forces, seismic forces, and tectonic forces. The casings prevent the collapse of the borehole wall and prevent the undesired outflow of drilling fluids into the formation or the inflow of fluids from the formation into the borehole. The borehole also may need to be cased due to equivalent circulating density and hydraulics reaching or exceeding the formation pore pressure or exceeding the fracture gradient pressure, which would allow fluids or gases to transfer between formations and borehole. If the formations are non-producing, or not of the desired producing interval (some intervals are producing but at low levels), the formations can be cased together. If shallow water flows (where water flows several hundred feet below the seabed floor), or if there is potential communication among formations, then the formation is cased. The casings extend downhole and are sequentially placed across the formations through which the wellbore or borehole passes. The casings may also be liners, which do not extend to the surface of the wellbore. Steel casing can be used to case off formations.
Each succeeding casing placed in the wellbore has an outside diameter reduced in size when compared to the casing previously installed, particularly to accommodate hangers for the inner strings. Thus, a well may be described as a series of nested casing strings. The borehole is drilled in intervals whereby a casing, which is to be installed in a lower borehole interval, is lowered through a previously installed casing of an upper borehole interval. As a consequence of this procedure, the casing of the lower interval has a smaller diameter than the casing of the upper interval. Thus, the casings are in a nested arrangement with casing diameters decreasing in the downward direction. The number of casings required to reach a given target depth is determined principally by the properties of the formations penetrated and by the pressures of the fluids contained in the formations.
Various types of casing may be installed in the well including structural or conductor casing, surface casing, intermediate or production casing, and production liners. Typically, a land based well starts with a 20″/18⅝″ or larger diameter casing and telescopes down through two or three intermediate casings, to a final casing size of typically 6⅜″ with a 5″ production liner installed. Each casing is secured in place with cement filling an annulus having a size typically varying from 1 to 10 inches over the length of the casing and may be as much as 14 to 21 inches or greater at a wash out in the borehole wall.
FIG. 1 is a schematic of a deep water well completion. The size and number of casing and tubing strings will increase or decrease depending upon the well plan based upon, for example, the depth of the well, the production tubing delivery size, the structural support and the seabed formation support. If the seabed formation is unconsolidated and has little support, then the structural or conductor casing is larger and is set deeper. If the initial conductor casing is in rock, then it can be smaller with substantially less depth. For example, initially a structural or conductor casing and riser are lowered from a drilling platform and driven, drilled or jetted into the sea floor to provide support for a surface casing. The structural or conductor casing may or may not be cemented.
FIG. 1 illustrates a 36 inch by 16 inch by 10¾ inch by 7 inch casing program with the addition of one or more tubing strings. After the 36 inch conductor casing is set, one or more surface casings is installed. A borehole is drilled for a 20 inch surface casing which is lowered into place with a 21″ surface casing riser attached thereto. A subsea wellhead with blowout prevention equipment, such as an 18¾ inch blowout preventer, is installed on the surface casing. The subsea wellhead may be supported by a structural casing.
Further, a borehole may be drilled through the riser and wellhead and through a problematic formation to extend a structural casing through the problem formation. For example, there are salt formations in the deepwater of the Gulf of Mexico. The structural casing forms a barrier across the formation while also supporting the wellhead. The structural casing has a thicker wall and provides a stable support frame for and can carry the load on the subsea wellhead. A 16 inch structural casing may be drilled, installed and cemented through a salt formation to seal off the salt formation from the wellbore being drilled. It should be appreciated that if there is no problematic formation, such as a salt zone, a shallow water flow zone, loss circulation zone, or other problem zone, then a structural casing is not needed to seal off the problematic area but it will support the subsea or platform wellhead, depending on well type.
Another borehole is then drilled for a 13⅜ inch intermediate casing string which is lowered into the borehole, attached to another riser, and cemented in place. Next a borehole may be drilled for another intermediate casing, such as a 11⅞ inch casing, and cemented in place. The borehole for the production casing string, such as a 9⅝ inch casing, is drilled and the production string is landed. It may or may not be cemented in place. The drilling is performed through blowout prevention equipment.
During the drilling of the wellbore, annuli are provided between the outer surfaces of the casings and the borehole wall and a composition, sometimes referred to as “oil field” cement, is introduced in the annulus for cementing the casing within the wellbore after the installation of each casing. When the casing is located in its desired position in the well, a cement slurry is pumped via the interior of the casing and around the lower end of the casing and upwards into the annulus, thereby causing the cement slurry to drive the drilling fluid upward in the annulus. As soon as the annulus around the casing is sufficiently filled with the cement slurry, injection of cement into the well is stopped and the cement slurry is allowed to harden, or cure. The cement sets up in the annulus, supporting and positioning the casing and forming a substantially impermeable that which divides the well bore into subterranean zones.
Ultimately the borehole reaches the target and is drilled through a hydrocarbon-containing formation or reservoir to produce hydrocarbons. The borehole may or may not be cased through the hydrocarbon-containing reservoir to allow substantially unrestricted influx of fluids from the formation into the borehole.
The purpose of the cement body around the casing is to fix the casing in the well. The cement also seals the borehole around the casing to prevent vertical flow of fluid alongside the casing towards other formation layers or even to the earth's surface. The cement prevents fluid exchange between or among formation layers through which the wellbore passes, and prevents the undesirable migration of fluids between zones or gas from rising up the wellbore.
It is important that there is no gas or fluid leakage after the cement has set and the well is completed. Thus, casing is cemented in place for two main reasons: (1) to seal off and prevent leak paths between permeable zones and/or surface; and (2) to give support and stability to the casings. A problem encountered during cementation of the casing is that due to various factors, such as the existence of varying pressure and temperature gradients along the length of the casing and shrinkage of the cement body during hardening thereof, relative displacements occur between the casing and the hardened cement that may result in poor bonding or cracking between the cement body and the casing.
Poor bonding may result in the presence of a so-called micro-annuli between the casing and cement body that may extend along a substantial part of the length of the casing. The occurrence of a micro-annuli is particularly dangerous in gas wells as substantial amounts of gas might escape to the surface. In some cases, hydrogen sulfide or natural gas can escape into the atmosphere. This condition may also lead to surface or ground water contamination. The resulting problems are very expensive to correct.
The poor bonding of the cement may be attributed to drilling fluid contamination or to bonding of the cement to the casing after the cement has set and/or oil or mill finish contamination on the surface of the casing. Poor bonding may also be contributed to aggressive drilling or aggressive pressure subjection and large pressure differentials prior to it hardening and during the operation. Also, hardening of cement generally causes a slight change of the volume of the cement.
A more fundamental cause of poor bonding is the loss of the hydrostatic head during the curing of the cement such that the formation pressure exceeds the annulus pressure and gas migration occurs causing channeling of the cement and subsequent leakage. For example, during cementing operations, it is common to both reciprocate and rotate the casing during the cement pumping operation to break up or close any cement channels around the casing. Also, compressible cement slurries have additives that entrain gas. During the cement pumping operation, the additives are compressed. As the hydrostatic head is lost during curing of the cement, the entrained gas subsequently expands and prevents loss of the pore pressure such that formation gas is prevented from migrating into the annulus. These specialized cement additives are expensive, however, and require specific operational techniques. This technique may also result in a lower strength cement. Additionally, too much of a hydrostatic head can be detrimental because the cement will take longer to cure and will be soft.
Additionally, heat of hydration, or the amount of heat formed by the curing of the cement, is an important design factor. If curing the cement produces too much heat, the physical characteristics of the casing and the formation can be affected. The properties of the cement itself may also be affected if the curing temperatures become too elevated.
Also, some cement slurries depend on the cement achieving high gel strengths in very short time periods. If there is a rapid static gel strength obtained, gas migration and channeling are reduced or prevented. The strength and capability of the cement to seal against fluid migration between various zones along the production casing is also an important design consideration.
Thus, it is essential that the cement slurry be designed to create a good bond between the cement body and both the casing and the borehole wall in the particular well environment. The various techniques and cement design factors must all be taken into account to form a proper bond between the casing and the formation. The cement must perform in the short term as well as long term through the life of the well. The cement formulation used for cementing wells will thus depend on the mechanical properties of the cement.